Mexico City -LRB- CNN -RRB- -- The runner-up in Mexico 's presidential election said Friday he still wo n't accept a vote count , even after the country 's electoral tribunal upheld the legality of the election and officially declared Enrique Pena Nieto as the winner .

Leftist candidate Andres Manuel Lopez Obrador has not conceded the July 1 election , citing allegations of electoral fraud by the victorious Institutional Revolutionary Party , known as the PRI .

The electoral tribunal on Thursday rejected a demand by Lopez Obrador 's coalition , the Progressive Movement of Mexico , to invalidate the presidential election , which he lost by 6 percentage points . On Friday , it validated the final results , officially naming Pena Nieto as president-elect .

`` The elections were not clean or free or authentic . As such , I will not recognize the illegitimate power that came from vote-buying and other grave violations of the constitution and the laws , '' Lopez Obrador said .

The candidate called on his followers to demonstrate in Mexico City 's historical square , the Zocalo , on September 9 .

Lopez Obrador narrowly lost another presidential race in 2006 . Back then , he refused to recognize the new government and called his supporters into the streets . He went on a national tour , where he drummed up support , calling himself the `` legitimate president of Mexico . ''

It was unclear if he would call for similar demonstrations now .

In a unanimous decision announced Thursday , the electoral tribunal ruled that the demand to invalidate the election was `` unfounded . ''

The tribunal found that the leftist coalition that brought the charges did n't prove any constitutional violations and did n't show that the process was n't free and fair .

Critics of the process said the irregularities included illegal campaign spending , secretive financing and coercion of voters .

Read more : Vote-buying allegations persist after Mexican election

When the allegations surfaced immediately after the vote , election officials recounted the votes in more than half of the ballot boxes individually .

Among the bigger controversies were allegations that PRI campaigners passed out hundreds of supermarket gift cards in exchange for votes .

The electoral tribunal said that the coalition could prove only that PRI officials gave the cards to supporters , and not to the general public in exchange for votes .

Pena Nieto is expected to be sworn in on December 1 .

CNNMexico 's Belen Zapata and Tania Montalvo contributed to this report .

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NEW : Mexico 's electoral tribunal declares Enrique Pena Nieto as president-elect

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NEW : He is expected to take office on December 1

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Andres Manuel Lopez Obrador says he will not accept the ruling

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He calls for a demonstration in Mexico City 's main square